Originally Posted by Orson Welles
I know that there has to be some critical trick to hitting the "tweener shot" (a.k.a., the "Sabatweenie") (between the legs with back facing the net).
There are NO 'magic cure' in learning tennis.
Rather than a 'short cut', tips are merely a 'map' that guide you from going the LONG (wrong) way.
Back to this shot:
1. TIMING : The ball MUST be contact with the space bounded by your legs.
2. WRIST : A loose wrist is a MUST to flick the ball at the correct angle.
3. LOFT : Flick at an angle to lift the ball to clear the net.
If you really want it, I'll suggest you to learn by progression:
1. Start from mid court with your back to the net. Drop feed yourself a ball and try to get it over the net.
2. Gradually move towards the baseline. Still drop feeding.
3. Toss the ball one step away from you, take a step and hit the shot.
4. Progressively toss the ball further and further away (you would also have to toss the ball higher, that goes without saying), take a few steps and hit.
5. Get a friend to lob while you run for the ball.
This is definitely not an easy shot to learn or do, let alone in a real game when you chase down a lob.